1. Field of the Invention
This invention relates to a rotary head type recording and/or reproducing apparatus and more particularly to an apparatus arranged to record and/or reproduce signals with a plurality of adjacently disposed heads on or from a recording medium and to permit selection of any of a plurality of recording medium transporting speeds.
2. Description of the Related Art
It has been known that a recording and/or reproducing apparatus of the kind handling digital signals of a high bit rate, such as video signals, must have a plurality of heads arranged on a rotary drum to be simultaneously in contact with a recording tape (medium) for the purpose of reducing the bit rate per channel by allotting the signal to a plurality of channels.
FIGS. 1 to 3 of the accompanying drawings show an arrangement for allotting the signal to four channels. Of these figures, FIGS. 1 and 2 show the allocation of the rotary heads in a top plan view and a side development view. In these figures, a reference numeral 1 denotes a rotary drum; a numeral 2 denotes a magnetic tape; and symbols A to H denote the heads respectively. As shown, the heads A to H are mounted on the periphery of the drum 1, with four heads spaced at a degree .THETA. and positioned stepwise at a stepped difference X on each of two opposite sides of the drum circumference. The heads come into contact with the magnetic tape 2 one by one accordingly as the drum 1 rotates. However, since at least four of them are simultaneously in contact with the tape 2, the signal can be recorded or reproduced in or from four channels.
When the magnetic tape 2 is conveyed at a given speed V.sub.T under this condition, a track pattern is formed with equal track pitches T.sub.P as shown in FIG. 3. A high degree of recording density is attained with the necessity of guard bands obviated by setting azimuth angles of +.alpha..degree. and -.alpha..degree. for adjacent heads in such a way as to utilize an azimuth loss.
Meanwhile, there is a desire for altering the bit rate of the digital signal to be recorded and reproduced. In the case of a video signal, the bandwidth of the signal can be compressed by various known methods. The amount of data is reducible without much deteriorating picture quality. This permits continuous recording or reproduction over a long period.
To meet this requirement with the arrangement described, therefore, it is conceivable to alter the bit rate for increasing the recording time by setting the tape speed at a 1/2 speed and by recording a signal simultaneously in two channels.
However, in a case where the four heads A, B, E and F are used, the track pattern becomes as shown in FIG. 4 (with the spacing angle .THETA. set at 13.5 degrees) and it is impossible to uniformly have the track pitches.
Assuming that the total number of heads is 2N, a track pitch obtained at a tape speed V.sub.T is T.sub.P, a track pitch T.sub.P ' obtained at a tape speed 1/n V.sub.T with two heads spaced at the angle .THETA. and at a stepped difference X in position becomes as expressed below (see FIG. 5): ##EQU1##
Referring to FIG. 5, an oblique one-dot chain line represents a head locus obtained at the tape speed V.sub.T and a broken line represents another head locus obtained at the tape speed 1/n V.sub.T. An intersection point between a vertical line p and these lines indicates the stepped difference X between the heads. Intersection points between another vertical line q and these lines indicate the actual track pitches T.sub.P and T.sub.P '. In other words, the track pitch decreases by a degree as expressed below: ##EQU2## This causes a decrease in output and results in deterioration in S/N ratio. Especially, in the case of recording a digital signal, this presents a serious problem, because: It is known that the deterioration of S/N ratio by 1 dB increases the rate of data errors by about ten times.
Further, an attempt to secure a desired S/N ratio at the narrow track pitch T.sub.P ', would result in wasteful use of the tape because the average track pitch is T.sub.P. In other words, the rate of the wasteful tape use would be T.sub.P /T.sub.P ' times as much as the normal use (1.15 times in the above example). This means a decrease in recording density.